# 三电平升压MPPT控制的三相光伏系统

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 5, SEPTEMBER 2008 2319Three-PhasePhotovoltaic System With Three-LevelBoosting MPPT ControlJung-Min Kwon , Student Member, IEEE, Bong-Hwan Kwon , Member, IEEE, and Kwang-Hee Nam, Member,IEEEAbstract — This paper proposes a three-phase photovoltaic (PV)system with three-level boosting maximum power point tracking(MPPT) control. A simple MPPT control using a power hysteresistracks the maximum power point (MPP), giving direct duty con-trol for the three-level boost converter. The three-level boost con-verter reduces the reverse recovery losses of the diodes. Also, aweighted-error proportional and integral (PI) controller is sug-gestedto control the dc link voltage faster. All algorithms and con-trollers were implemented on a single-chip microprocessor. Exper-imental results obtained on a 10-kW prototype show high perfor-mance, such as an MPPT ef?ciency (MPPT effectiveness) of 99.6%,a near-unity power factor, and a power conversion ef?ciency of96.2%.Index Terms— Maximum power point tracking (MPPT), photo-voltaic (PV) system, three-level boost converter.I. INTRODUCTIONE NVIRONMENTAL concerns about global warming,fossil fuel exhaustion, and the need to reduce carbondioxide emissions provide the stimulus to seek renewable en-ergy sources. Speci?cally, solar energy hasthe advantagesof nopollution, low maintenance cost, no installation area limitation,and no noise due to the absence of the moving parts. However,high initial capital cost and low energy conversion ef?ciencyhave deterred its popularity. Therefore, it is important to reducethe installation cost and to increase the energy conversionef?ciency of photovaltaic (PV) arrays and the power conversionef?ciency of PV systems.PV arrays are known to be nonlinear, and there exists one op-erating point where the PV array generates maximum power.In order to achieve maximum utilization ef?ciency of the PVarray, the MPPT control technique, which extracts the maximumpossible power from the PV array, is essential. Various MPPTcontrol methods have been proposed, such as the lookup tablemethod [1], [2], incremental conductance (IC) method [3] – [6],and perturb-and-observe (P revised April 23, 2008. Currentversion published November 21, 2008. Recommended by Associate Editor R.Teodorescu.The authors are with the Department of Electronic and Electrical Engineering,Pohang University of Science and Technology, Pohang 790-784, Kyungbuk,South Korea (e-mail: jmkwon@postech.ac.kr; bhkwon@postech.ac.kr;kwnam@postech.ac.kr).Color versions of one or more of the ?gures in this paper are available onlineat http://ieeexplore.ieee.org.Digital Object Identi?er 10.1109/TPEL.2008.2001906IC method and P&O method have an advantage of not requiringsolar panel characteristics. The IC method uses the PV array ’sincremental conductance . At the MPP, it utilizes an ex-pression derived from the condition . This methodprovides good performance under rapidly changing conditions[3] – [5].The P&O method perturbs the operating voltage of thePV array in order to ?nd the direction change for maximizingpower. If power increases, then the operating voltage is furtherperturbed in the samedirection, whereas if it decreases,then thedirection of operating voltage perturbation is reversed [6] – [9].This paper suggests a simple MPPT method for the three-levelboost converter. This MPPT control uses power hysteresis totrack the MPP, giving direct duty control.As a conventional PV system, a single-stage inverter withtransformer is widely utilized. Its circuit has advantages in thatmuch more utility grid-tie voltage options canbe selected by se-lecting different turns ratios of the isolation transformer. How-ever, the transformer lowers the overall power ef?ciency and in-creases the cost and the size [10]. Recently, two-stage PV sys-tems have been proposed without the bulky 50/60 Hz step-uptransformer [11] – [15].These transformerless PV systems havethe advantages of small size and reduced cost.This paper proposes a three-phase PV system composed of athree-level boost converter and a three-phase inverter as shownin Fig. 1. The three-level boost converter reduces the switchinglosses and the reverse recovery losses. The interleaving tech-nique is utilized for the three-level boost converter to reducethe input ?lter size by input current ripple cancellation. Also,EMI is lower since the PWM actions are happening betweenhalf output voltages. A weighted-error PI controller is suggestedfor fast dc link voltage control. All control functions are im-plemented fully in software with a single-chip microprocessor.Thus, the three-phase PV system is realized with minimal hard-ware and at low cost. Experimental results obtained on a 10-kWprototype show high performance, such as wide range of the PVvoltage, high MPPT ef?ciency (99.6%), high power conversionef?ciency (96.2%), a near-unity power factor, and low currentTHD (2.0%).II. SYSTEM CONTROL AND A NALYSISThe proposed PV system is composed of the three-level boostconverter and the three-phase inverter. The three-level boostconverter performs MPPT control and also gives step-up func-tion of the PV voltage. The three-phase inverter regulates the dclink voltage and generatesthe ac power. Thus, the three-phaseinverter performs a step-down function. The power converterwith stepup/down function allows a wide range of PV voltages.The three-level boost converter has several advantages in highvoltage applications such as reduced switching losses and re-duced reverse recovery losses of the diode [16] – [18].The high0885-8993/$25.00 ? 2008 IEEE2320 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 5, SEPTEMBER 2008Fig. 1. Proposed three-phase PV system.voltage rated IGBT used in the conventional boost converter hasa larger on-drop voltage than the low voltage rated IGBT. How-ever, the IGBT used in the three-level boost converter has halfthe rating of that used in the conventional boost converter. As-suming the same output capacitance for devices with differentvoltage ratings, the capacitive turn-on loss of the three-levelboost converter is reduced eight times. The reverse recoverylosses of the diode are also reduced, since the reverse voltageis half of the output dc link voltage, and the diodes with halfvoltage rating are faster.A. Maximum Power Point TrackingThe P&O method has an advantage of not requiring solarpanel characteristics asinputs andbeing easyto implement. Thispaper suggests a simple P&O method for the three-level boostconverter. The MPPT process and its ?owchart are shown inFigs. 2 and 3, respectively. The denotes the duty directionfor MPPT. The PV voltage is decreasedwhen the is 0 andincreased when the is 1. Therefore, in Fig. 2, the s ofpaths , , and are 0, andthe s of paths and are1. The starting point is the open circuit voltage point. At startupof the MPPT control, the , , , and are ini-tialized as follows:(1)The current PV power is calculated by the PV voltageand the PV current , which are averagedduringthe MPPT control period. After the startup of the MPPT con-trol, the operating point moves to MPP through the path and. After the operating point reaches the MPP, the operatingpoint varies between point and point . Point and pointaredetermined by the comparative power for the MPPTdirection. The comparative power has ahysteresis char-acteristic as follows:(2)Fig. 2. MPPT process.where is the PV power updated at the previous cycle andis the power hysteresis for perturbing the power variation.Finally, the duty ratio direction of the three-level boost converteris directly determined by the . The duty ratio is increasedwhen the is 0 and decreasedwhen the is 1.KWON et al. : THREE-PHASE PHOTOVOLTAIC SYSTEM WITH THREE-LEVEL BOOSTING MPPT CONTROL 2321Fig. 3. Flowchart of the MPPT control.B. Three-Level Boost Converter and DC Link VoltageBalancing ControlWhen the duty ratio is less than 0.5, the waveforms of thethree-level boost converter are shown in Fig. 4(a). Prior to ,switches and are turned off. At , which is the begin-ning of a switching cycle, the switch is turned on and thecurrent ?ows through , , , and . The PV currentincreases as follows:(3)where(4)At , the switch is turned off and both switches are notconducting. The current ?ows through , , , , and, and the PV current decreasesas follows:(5)At , the switch is turned on and the current ?ows through, , , and . The PV current increases like as(3). At , the switch is turned off and both switches are notconducting. The current ?ows through , , , , and. The PV current decreases,as in (5).When the duty ratio is grater than 0.5, the waveforms of thethree-level boost converter are shown in Fig. 4(b). Prior to , theswitch is turned off andthe switch is turned on. At , theFig. 4. Theoretical waveforms of the three-level boost converter. (a) D0 :5 . (b) D > 0 :5.switch is turned on and both switches are conducting. ThePV current increases like a conventional boost converter(6)At , the switch is turned off and the current ?ows through, , , and . The PV current decreases as fol-lows:(7)At , the switch is turned on and both switches are con-ducting. The PV current increases again, as in (6). At ,the switch is turned off and the current ?ows through ,, , and . The PV current decreasesas in (7).Since the capacitors and are alternatively charged,their voltages and are theoretically balanced. In re-ality, they are not since the parameters of the components arenot exactly balanced. To ensure equal voltages of the two capac-itors and , a voltage balancing controller is essential.Fig. 5 shows the controller of the three-level boost converter2322 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 5, SEPTEMBER 2008Fig. 5. DC link voltage balancing controller.that ensures equal balancing of the dc link voltages and. The duty ratio of the boost switch is determined bythe MPPT control and the duty ratio of the boost switch isdetermined by adding an additional duty for the dc link voltagebalance . The additional duty for current balanceis(8)where is the integral control gain of the dc link voltagebalancing controller.The total dc link voltage is regulatedby the inverter. As the dc link voltage is increased, the in-verter increases the output power. On the other hand, as the dclink voltage is decreased, the inverter decreases the outputpower.C. Fast DC Link Voltage ControlIn Fig. 1, , , and are the grid voltages and , ,and are the output leg voltages. , , and are the outputcurrents. The voltage equations in the stationary frameare(9)where and are the maximum phase voltage and angularfrequency of the grid, respectively. The voltage equations in thestationary frame are given by(10)The voltage equations in the synchronous frame are givenby(11)where and are the -axis output voltage and current, andand are the -axis output voltage and current. The gridvoltages of -axis are and . For a unity powerfactor, it is desirable that the -axis current is zero. Then the-axis current is controlled with the zero reference current. The active power supplied to the grid is(12)Since the active power is directly proportional to the -axiscurrent , the -axis reference current is generated from thePI voltage controller for the dc link voltage regulation. The con-ventional PI voltage controller is(13)(14)where and are the reference dc link voltage and the dclink voltage, and are the proportional and integral controlgains of the PI voltage controller, and is the error voltage be-tween and . The regulated dc link voltage is an importantfactor for achieving high performance. However, the conven-tional PI voltage controller controls the dc link voltage slowlyand the dc link voltage variation exists. To reduce the dc linkvoltage variation, a weighted-error PI voltage controller is sug-gested. The suggested controller is(15)(16)where is the weighted-error between and , and isthe weighting scale factor. Compared to the error term of theconventional PI voltage controller in (14), theterm is included. Fig. 6 shows the difference of the error termof the conventional PI controller (14) and the error term of thesuggested weighted-error PI controller (16). The horizontal axisrepresents the difference of and , and the vertical axisrepresents the weighted error . Due to the term ofin error term of the suggested controller, a large errorbetween and increases the weighted-error more thanthe error voltage of the conventional PI controller . Therefore,if the voltage error is large, then the dc link voltage is con-trolled rapidly as the PI controller has a large gain. In contrast,if the voltage error is small, the suggested controller gives al-most the same characteristic as the conventional PI controller.Since the stability problem basedon the small-signal analysis ishandled with almost zero voltage error, the stability of the pro-posed weighted-error-based PI controller is almost the same asthe conventional PI controller.D. Current Controller for Unity Power FactorThe voltage equations (9) aretransformed from the stationaryframe to the synchronous frame as follows:(17)KWON et al. : THREE-PHASE PHOTOVOLTAIC SYSTEM WITH THREE-LEVEL BOOSTING MPPT CONTROL 2323Fig. 6. Difference of (a) the error term of the suggested weighted-error PI con-troller ( = 10) and (b) the error term of the conventional PI controller.To make the input currents track the reference currents, the PIcurrent controllers can be utilized. However, the PI current con-trollers do not work well as rapid tracking controllers for thecoupled system in (17). To avoid this problem, the followingdecoupling control is effective:(18)With the addition of the overall current controller (18) to the in-verter (17), which is originally a coupled dynamic system, theinput-output relations of the inverter become ?rst-order decou-pled linear dynamic systems with easy controllability as fol-lows:(19)The output signals and of the current controllers gen-erate transient additional voltages required to maintain the sinu-soidal input currents(20)and are proportional control gains and and areintegral control gains. Thus, the overall current controller in thesynchronous reference frame relaxes the burden of the PI cur-rent controllers and improves the input current waveform. Fig. 7shows the decoupled control diagram for the inverter system.The PWM pulses of the three-phase inverter aregenerated bythe space-vector modulation (SVM) technique. The SVM tech-nique is a popular PWM method for the three-phase inverterwith isolated neutral load because of two excellent features:Its maximum output voltage is 15.5% greater and the numberFig. 7. Decoupled control diagram for three-phase inverter.of switching is about 33% less at the same carrier frequencythan the ones obtained by the sinusoidal pulse-width modula-tion method. An effective software implementation of the SVMfor current control on the rotating frame [19] is adopted.III. EXPERIMENTAL RESULTSThe hardware circuit of the three-phase PV system inFig. 1 is